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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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GLACIATION OF PRINCE WILLIAM SOUND                   483
because of its great depth and breadth. Ice confined in narrow mountain valleys, and flowing through them with rapidity has quite generally been accredited with great erosive power; but ice which, emerging from narrow valleys, spreads out in a broadly-open depression, loses that velocity and concentration upon which effective ice erosion depends. While at first sight this seems to be a description of Prince William Sound, on closer study it is found to be not a strictly correct description. It is true that there are numerous, narrow, ice-eroded fiords, it is true that Prince William Sound is a broadly open depression, and it is true that there are great depths of water in it. But the apparent size of the depression is diminished by the fact that the western half is largely occupied by islands; and the apparent depth is less significant when it is noted that the deep parts are mainly, if not entirely, along rather narrow submarine channels (Fig. 72), in which it is a reasonable hypothesis to assume that the ice movement was more vigorous than elsewhere.
A fact of importance bearing on this problem is that the tributary valleys—the fiords— themselves have tributary hanging valleys at such elevations as to prove that before the ice invasion-the main valley bottoms were well above the present sea level. This being the case, it is inconceivable that a few miles down their courses such valleys should become so deep as the bottom of Prince William Sound, for this would require an increase in valley depth of about 1500 feet in fifteen or twenty miles. Two hypotheses are possible to account for such valley discordance, (1) orographic changes, (2) differential glacial erosion. Of the former there is not only no evidence, but there are grave difficulties in accounting for the peculiar distribution of excessive depths, which harmonize with valleys and probable direction of ice motion and show no indication of origin by folding or faulting. In favor of glacial action is the fact that the ice was here for a long time, giving a cause known to exist, and the harmony of depths with lines of probable ice flow.
While we do not consider this explanation definitely established, we propose the following as an hypothesis for the conditions in the Prince William Sound region. Before the advent of the glaciers there existed here a mountain valley system tributary to the lowland of Prince William Sound, whose bottom was then at a level unknown at present but probably slightly above the present sea level. The glacial occupation gave rise to differential ice erosion, deepening and widening the fiords, excavating rock basins, and eroding to variable degree throughout the main Prince William Sound, but especially along the lines of most rapid flowage, namely out of the fiord mouth and down the middle of the eastern half of the sound. We believe, therefore, that Prince William Sound and the tributary fiords have been produced without sinking of the land.
Deposits of the Expanded Glaciers. In the Yakutat Bay region the expanded glaciers have left us well-marked records of both their outer stand and their recession. In our studies of Prince William Sound we have found no such distinct records. As stated above, the only interpretation that we can place upon the absence of terminal deposits is that these lie hidden from view in Prince William Sound and on the continental shelf south of the mouth of the sound.
Extensive recessional gravels, like those plainly visible in Russell Fiord, are not found in Prince William Sound. It is possible that this apparent absence of glacial deposits may be due to the fact that our studies here were not extensive enough, or that the forest covering has obscured the evidence, though this does not seem probable. If glacial de-